1. Field of the Invention
This invention relates to surface-launched acoustic wave sensors, such as chemical and biological sensors, and more particularly to increased temperature stability thereof.
2. Description of the Prior Art
As is known in the art, surface-launched acoustic wave devices may be utilized to detect and quantify numerous measurands by means of perturbations induced in the electrical and mechanical properties of the devices by those measurands. Surface-launched acoustic wave devices include several specific classes, such as surface acoustic waves (SAW), surface transverse waves (STW), surface skimming bulk waves (SSBW), pseudo-surface acoustic waves (PSAW), acoustic plate modes (APM), Love waves, Lamb waves, and liquid guided acoustic waves (LGAW). Acoustic waves can be generated and measured using interdigital transducers on the surfaces of piezoelectric substrates, such as quartz, lithium niobate, lithium tantalite, etc., whereby an electric potential is converted into a mechanical strain and vice versa. The specific geometry of the substrate and interdigital transducers and the type and crystallographic orientation of the substrate material determine the spectrum of waves that will be excited and measured. Through careful control of these parameters and the environment in which the device is operated, it is usually possible to realize a device that allows the generation, propagation, and detection of only one class of the surface-launched acoustic waves listed above (e.g. SAW).
Because the velocity of an acoustic wave is sensitive to the mechanical and electrical properties of the medium through which it propagates, surface-launched acoustic wave devices can be used as sensors, as taught, for example, by U.S. Pat. No. 4,312,228 as shown in FIG. 1. Velocity of the waves can be measured using a delay line oscillator configuration. This configuration consists of (1) an input interdigital transducer which converts an electrical signal into a mechanical wave, (2) a finite distance, denoted the xe2x80x9cdelay pathxe2x80x9d, through which the mechanical wave propagates, (3) an output interdigital transducer which converts the mechanical wave back into an electrical signal, and (4) an amplifier which increases the output electrical signal strength and feeds the stronger signal back into the input interdigital transducer. Because of this positive feedback configuration, self-oscillation is sustained at a specific frequency determined primarily by the velocity of the mechanical wave. Therefore, the oscillation frequency can be used as a direct measure of the velocity.
Likewise, an acoustic resonator configuration can be used to measure velocity of the acoustic wave. In this configuration, one or two interdigital transducers are disposed upon a piezoelectric substrate so as to form an acoustic resonant cavity. Using appropriate resonator electronics, an oscillator circuit can be realized, whereby the oscillation frequency thereof can be used as a direct measure of the velocity.
A chemical sensor can be realized by coating the device or (in the case of a delay line) the delay path thereof, with a thin coating which is physically, chemically, biologically, or otherwise sensitive to a target analyte (measurand). Mechanical and electrical perturbations to the thin coating induced by interaction with the target measurand, including changes in surface-bound mass, elasticity, electrical conductivity, and permittivity, alter the acoustic wave velocity and are, therefore, manifested as alterations in the oscillation frequency of the device. Oscillation frequency can then be correlated to concentration or quantity of the target analyte. Hence, a surface-launched acoustic wave sensor is realized.
One of the key limiting factors to the ultimate sensitivity of a surface-launched acoustic wave sensor is noise induced by inevitable temperature fluctuations. Temperature changes induce noise in the sensor""s operating frequency for two major reasons. First, the elastic, dielectric, and piezoelectric coefficients of the substrate change with temperature, thereby changing the velocity of the wave and, hence, the oscillation frequency. The magnitude of this effect is called the xe2x80x9ctemperature coefficient of velocityxe2x80x9d (TCV). Second, the substrate grows or shrinks with changing temperature. This changes the distance across which the acoustic wave must propagate, thereby changing the net phase shift through the device. This results in a slight change in resonant frequency to offset the phase shift. The magnitude of this effect is denoted the xe2x80x9cthermal expansion coefficientxe2x80x9d, xcex1. The overall temperature sensitivity of the oscillation frequency, or xe2x80x9ctemperature coefficient of frequencyxe2x80x9d (TCF), is given as:
TCF=TCVxe2x88x92xcex1xe2x80x83xe2x80x83(1)
The larger the magnitude of TCF, the more sensitive the device is to temperature fluctuations. Two strategies are commonly employed to reduce this temperature noise. The first is to minimize TCF by utilizing a substrate for which TCV and xcex1 are exactly equal at some temperature and, hence, subtractively cancel in equation 1. For SAW applications rotated Y-cut quartz is a common example of such a substrate. By varying the rotation angle of the cut, one can choose any temperature over a broad range for which the substrate will be stable. FIG. 2 shows the theoretical variation of normalized oscillation frequency as a function of temperature for 42.75xc2x0 (also denoted ST cut) and 26xc2x0 rotated Y-cut quartz. As the figure shows, the oscillation frequency for each of these cuts (and all others in the rotated Y-cut family) demonstrates a parabolic behavior such that frequency increases with increasing temperature up to some xe2x80x9cturnover temperaturexe2x80x9d (the vertex of the parabola), whereby further temperature increases result in a decrease in oscillation frequency. For small temperature variations around the turnover temperature, TCV will balance a in equation 1 above, and frequency fluctuations will be minimized. Again, as shown in the figure, different rotation angles result in different turnover temperatures.
While temperature compensated substrates are used commonly for noise reduction in acoustic wave signal processing applications, where it is desired to maintain a constant frequency at all times, this technique is not as practical for sensor applications. In sensor applications, one typically coats a portion of the acoustically active area of the device with a material that facilitates sensitivity to a target analyte. The very application of this material, however, can dramatically alter the temperature characteristics of the device. For applications where many different film materials are to be used or applications where film parameters such as thickness, viscosity, electrical conductivity, or density may change over time, it is often impractical or impossible to maintain adequate temperature stability.
The second strategy commonly used to reduce temperature noise is the xe2x80x9cdualxe2x80x9d configuration, where a reference oscillator (channel) is used in tandem with the sensing channel, as taught, for example, in U.S. Pat. No. 5,992,215 as shown in FIG. 3. The reference channel is located on the same substrate and is designed to be exactly the same as the sensing channel, but without the sensing film (coating). The oscillation frequency of this reference is then subtracted from that of the sensing channel. Temperature fluctuations that affect both channels equally are, therefore, cancelled through subtraction. Ideally, this results in a signal that is dependent only on changes in the sensing film.
However, in reality this is not the case. As described above, the addition of any perturbation to an acoustic device (including a sensing film) can dramatically affect the TCF. Therefore, temperature will not have the same effect on the sensing channel as it does on the reference channel, and the temperature noise will not be cancelled out. In some cases, it can even be made worse. For example, FIG. 4 shows the theoretical difference in oscillation frequency for two surface acoustic wave devices on the same substrate (24xc2x0 rotated Y-cut quartz) as a function of temperature. The only difference between the two devices is the addition of a thin metal film on one. As shown in the figure, however, this film has a striking impact on the temperature sensitivity of the device. Hence, the difference frequency of such a dual setup would still be highly temperature sensitive, defeating the whole purpose of this strategy.
It is therefore an object of the present invention to mitigate temperature-induced measurement noise in surface-launched acoustic wave sensors with a novel strategy for matching the temperature characteristics of two or more delay lines or resonators, one being sensitive to a target analyte and the other acting as a reference to which the former is compared.
According to one embodiment of the present invention, the above is accomplished by utilizing an impermeable layer of chemically inert material on both channels. On the sensing channels, this impermeable passivation layer is deposited beneath the sensing films and, therefore, has essentially no effect on the chemical sensitivity of the device. On the reference channel, however, the passivation layer is deposited on top of a sensing film identical to that on the sensing delay line, thus rendering that film insensitive. Because both delay lines contain the same two films, albeit in different orders from top to bottom, the temperature characteristics of the two delay lines are essentially identical. This remains true for any number of films in any order, as long as the cumulative thickness of the films is less than the acoustic wavelength (typically tens of microns). Thus, this invention is based on the principle that the order in which different thin films are deposited upon a surface-launched acoustic wave delay line has virtually no bearing on the temperature characteristics of that delay line, but it plays a pivotal role in determining the chemical sensitivity of the device.
According to another embodiment of the invention, the impermeable passivation layer is extended to cover the entire acoustically active portion of each interdigital transducer. This impedes oxidation and other chemical changes to the metal interdigital transducers, thereby reducing long-term drift of the oscillation frequency. Electrical contact to the interdigital transducers is then made by extending the bus bars away from the acoustically active area.
In a third embodiment of the invention, one or more serpentine resistive metal strips are added to the device in order to precisely measure temperature at the surface of the device. The passivation layer is extended to cover this resistive thermal device (RTD), thereby eliminating long-term drift in the resistance due to oxidation and other chemical interactions. By coupling this device with a heater and using the RTD for temperature feedback, temperature of the device can be controlled very accurately, further reducing temperature-induced noise in the oscillation signal. This embodiment of the invention may be used in a SAW dual delay line sensor arrangement on a quartz substrate, with gold as the sensing film and glass (SiO2) as the passivation layer.